DISPERSAL BY WATER. 809 



plants brought by birds; their roots are either not hxed to the ground but sway 

 about in the water, or they may be altogether absent; examples are, Riccia 

 fluitans and R. nutans, Lemna and Wolffia, and in tropical regions Azolla and 

 Pistia. All these multiply very rapidly. While they continually branch at one 

 end, forming spreading lobes and sprouts, they die away on the other, the result 

 being of course a separation into several pieces, i.e. into offshoots. These fragments 

 spread themselves like a green mosaic over the surface of the water. As the off- 

 shoots increase in numbers a certain number of them will extend beyond the calm 

 inlet by the bank into the flowing water in mid-stream. Here they are hurried 

 away by the current, and it often happens that they travel some distance before 

 they are again stranded in some calm spot near the bank to form again the starting- 

 point of a fresh aggregate of offshoots. 



Rain-water also plays an important part in the distribution of offshoots. Those 

 of the widely spread Liverwort, Marchantia polymorpha, so frequently met with 

 on damp earth, are especially noticeable in this respect. Their development is 

 represented in fig. 196, p. 23. On the surface of the dark-green leaf-like thallus of 

 this Liverwort cups arise, at the base of which papillae give origin to plate-like 

 brood-bodies (gemmffi, cf. figs. 196 ^ and 196 ^). Other papillte behave differently, 

 and undergo only slight enlargement. The heads of these latter then swell up 

 forming a gelatinous mass, and as this swells up it raises the green gemmaa higher 

 and higher out of the bottom of the cup (fig. 196^). At last they get close to the 

 edge and are washed out of it by the rain. The offshoots of other Liverworts are 

 also chiefly distributed by rain-water, as for instance the gemmse which arise 

 in the crescent-shaped pockets of Lunularia, and in the flask-shaped cavities of 

 Blasia pusilla. The pairs of cells which arise on the upper surface of Aneura 

 Tnultifida, the single cells which become detached from the edge of the fronds of so 

 many Liverworts, the multicellular offshoots which are given off by Radula com- 

 planata so common on the bark of trees, the round cell-plates growing on the edge 

 of the leaf -like thallus of Metzgeria pubescens, and finally the ball- and disc-shaped 

 groups of cells which develop on the surface of the leaves of numerous Mosses 

 (e.g. on various species of the genera Leucobryum, Grimmia, Zygodon, Ortho- 

 trichum, Barbula, Galymperes). In many of these cases the small offshoots are 

 detached as well as distributed by the action of rain-water, but in others the loosen- 

 ing occurs before the rain begins, and in Blasia and Aneura, as well as in 

 Marchantia, the offshoots are first separated by mucilaginous membranes, and are 

 thus raised up from their attachment. Not until afterwards are they washed out 

 and distributed by the falling rain. These small offshoots can of course also be 

 carried away from their place of origin by strong gusts of wind. Even breathing 

 strongly on them is sufficient to detach the uppermost gemmas of Marchantia, but 

 in dry air and in dry soil they rapidly shrivel up and perish. The distribution 

 by currents of air is therefore not attended by success, but the offshoots of the 

 Liverworts and Mosses washed out by showers of rain immediately begin to grow, 

 and quickly attain to further development. This mode of distribution plays an 



